Electrophoretic enamelling is a well-known method for coating the surface of electrically conductive objects, which may consist, for example, of metal or of conductive plastics or plastics that have been made conductive. In this method, the object to be coated is immersed in an aqueous coating bath and, in the case of cathodic deposition, connected as cathode to a source of direct current. The current flowing causes the coating material or enamel to coagulate and be deposited on the surface of the material. The firmly adhering material is then physically caused to flow by air drying or heating and, if necessary, cross linked chemically in order to produce a homogeneous, smooth, resistant surface.
The method of electrophoretic enamelling also permits cavities, which partly are difficult to reach, to be coated. It is therefore an objective of the optimization of the method to achieve as comprehensive as possible a coating of the cavities. A different objective is to achieve a smooth and homogeneous distribution of layers on the outer surfaces.
Different parameters can be varied in order to meet these requirements. In general, a better throwing power, that is, an improved coating of cavities is to be achieved by increasing the deposition voltage. However, the deposition voltage can be increased only within certain limits, which depend on the vehicle system selected, as otherwise coating disorders in the form of break-throughs which also lead to optically disturbing differences in layer thickness, occur on the sides next to the counterelectrodes of the cathode. One possibility of improving the distribution of thicknesses on smooth substrate surfaces is to increase the deposition temperature. However, it has turned out that the breakthrough voltage of the coating material is also affected negatively by this method. Moreover, because of the coating installations used in industry, there is a certain temperature range, in which it is possible to work with the least possible interventions from the outside and in which the heating or cooling energy that must be employed is most advantageous.
A further parameter is the degree of neutralization of the resin material used. The deposition of enamel is most advantageous if the addition of acid to the coating bath is as low as possible, that is, if the MEQ value (milliequivalents of acid per 100 g of solids) is low. Due to the current flowing, hydroxyl ions are produced at the cathode. These hydroxyl ions then neutralize the dissociated acid molecules present on the micelles of the coating bath. It is therefore understandable that an only slight addition of acid reduces the amount of hydroxyl ions necessary to precipitate the vehicle and that a lower current consumption per quantity of enamel deposited can thus be achieved. At the same time, the throwing power of the coating process is affected positively. The reduction in the amount of acid is limited by the need to produce a stable coating bath, which does not precipitate spontaneously under the conventional operating conditions.
If coagulates are formed, they are also deposited on the enamel surface and interfere with the quality of the film, since these particles no longer merge homogeneously. In addition, the coagulate interferes with other facilities present; for example, filters and pipelines are blocked and ultrafiltration facilities are adversely affected.
A further parameter having an effect on the distribution of layers and the coating conditions is the solvent content of the bath material. By the addition of small amounts of as far as possible organophilic, hydrophobic solvents, the thickness of the layer deposited is increased under the same coating conditions or the coating conditions can be changed in order to achieve a constant layer thickness. In addition, the enamel merges better during the cross linking process. The bath is adversely affected only insofar as the different micelles evidently form coagulates and impurities more easily due to the higher solvent content. These coagulates and contamination must be removed or the stability of the enamel must be increased, for example, by a greater mutual rejection of the particles. This can also be achieved by increasing the neutralization of the micelles.